You will love it. It's addictive. I started doing Body Pump 2 years ago, and now do Body Combat, Body Balance, Body Jam and Body Blast!! The instructors are all really well trained, the music is really modern and good to get you going. The classes are always full of a good range of people.
The first session will fly buy, but make sure you use low weights for the entire session, the following weeks gradually you will be able to lift more and more.
Make sure you tell the instructor you are new to the class they will teach you all the moves, there is only 1 complicated one being the "clean and press", which once you have grasped is really easy. You don't really need to be too coordinated either.
After your first session you will feel it in your legs, generally 2 days after, but once you have been going for a few weeks you won't notice anyhting.
You are actually starting at a really good time, they have just released a new body pump CD, they do this every few months, which means all the routines and music will be new to all the class members.
Enjoy it. x